The minimum pit aperture diameter of a vessel-parenchyma pit pair was found as the decisive wood anatomical feature for vessel occlusion by either tyloses or gums. Based on this observation, as well as on considerations and established knowledge about the functional significance of vessel occlusions, an evolutionary hypothesis is presented. In order to withstand microorganisms and embolisms, plants are able to occlude their vessels with tyloses or gums. The most primitive Angiosperms show tylosis formation. With the decreasing pit sizes in the vessel wall during plant evolution, gum formation was developed, whereas the increasing vessel sizes led in some plants to renewed tylosis formation.

Host responses of elms susceptible and resistant to Dutch elm disease were histologically examined. In a time course study the susceptible elm clone Ulmus × hollandica 'Belgica' and U. × hollandica '390', a clone which shows a high degree of resistance to non-aggressive isolates and a moderate degree of resistance to aggressive isolates of Ophiostoma ulmi, were inoculated in twig or trunk with either an aggressive or a non-aggressive isolate of O. ulmi. For purposes of comparison, the susceptible elm U. americana and the more resistant clones U. × hollandica 'Groeneveld', U. 'Lobel' and U. 'Sapporo Autumn Gold' were included. Depending on clone-isolate compatibility, infected twigs reacted by a walling off process, by barrier zone formation, or failed to resist the infection and died. Trees inoculated into the trunk reacted comparably but in the case of a compatible combination they always formed a barrier zone and the cambium never died in the year of inoculation.

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Additional layers on the inner vessel walls and on perforation plates in 'pathological' xylem parts were EDX-analysed. The layers contain high amounts of calcium. The origin of these layers in relation to bacterial activity is discussed.